It is useful to think of printed images as being constructed of printer elements, or pels. A pel is sometimes referred to as a dot, and the resolution of a printer is often characterized by the number of dots that it can print within a given linear distance, such as 1,200 dots per inch.
Laser-based electrophotographic printers typically form images by scanning a laser across the surface of a drum. The laser is pulsed on and off as it scans across the surface of the drum. Those portions of the drum surface that are struck by the beam of the laser undergo a physical change that enables the drum to pick up and then place toner on a sheet of paper.
To form a dot, the laser may be pulsed on and then off again for a given length of time within the pel. Often, a fifty percent duty cycle is used, meaning that the laser is off for approximately half of the total scan width of the pel, and on for approximately half of the total scan width of the pel. For example, the laser may be off as it scans across the first quarter of the pel, then pulsed on as it scans the middle half of the pel, and then pulsed off again as it scans across the final quarter of the pel. If a duty cycle greater than about fifty percent is used, the area of the drum surface affected tends to be greater in size than desired, producing a dot of a size that is larger than typically necessary.
Among the general design goals of printers are those of increased speed and increased resolution. Increasing printer speed means to decrease the amount of time required to print a page, or in other words, increase the scan speed of the laser. Increasing the resolution of the printer means to decrease the size of the pels, or in other words, put more pels within a given linear distance. Achieving either of these goals results in the laser operating at an increased frequency. For example, if the scan speed of the laser is increased, then it forms more pels per unit time. Since the laser typically is pulsed on and off again for each pel as described above, the laser is pulsing on and off more frequently per unit time. Similarly, if the resolution increases, then more pels are formed per unit scan distance of the laser. Assuming that the laser scan speed is not reduced, this also means that the laser is pulsed on and off more frequently per unit time. Increasing both speed and resolution at the same time only compounds the situation.
The frequency at which the laser operates tends to be related to the electromagnetic interference (EMI) that is produced by the printer. Typically, increasing the operating frequency of the laser tends to increase the EMI produced by the printer, and any reduction in the operating frequency tends to decrease the EMI. Other factors which tend to affect EMI are the amount of common mode current present on the cable which connects the laser to the laser controller, and the length of that cable.
Governments regulate the amount of radiated EMI that a device such as a printer may emit. Thus, the amount of EMI produced by a printer is of great concern to printer manufacturers. Some manufacturers have reduced the length of the cable carrying the laser control signal to reduce EMI, or reduced the voltage of the laser control signal to the laser, which tends to reduce the current. Other techniques include using coaxial cable or adding toroids to the cable. These methods tend to reduce the antennae effects of the cable, but they tend to be expensive and are typically used as a last resort to save the product schedule. What is needed, therefore, is a method of reducing EMI produced by a printer, without significantly increasing the production cost or reducing the speed or apparent resolution of the printer.